Given two numbers represented by two lists, write a function that returns sum list. The sum list is list representation of addition of two input numbers.

Example 1

Input: First List: 5->6->3 // represents number 365 Second List: 8->4->2 // represents number 248 Output Resultant list: 3->1->6 // represents number 613

Example 2

Input: First List: 7->5->9->4->6 // represents number 64957 Second List: 8->4 // represents number 48 Output Resultant list: 5->0->0->5->6 // represents number 65005

**Solution**

Traverse both lists. One by one pick nodes of both lists and add the values. If sum is more than 10 then make carry as 1 and reduce sum. If one list has more elements than the other then consider remaining values of this list as 0. Following is the implementation of this approach.

## C

#include<stdio.h> #include<stdlib.h> /* Linked list node */ struct Node { int data; struct Node* next; }; /* Function to create a new node with given data */ struct Node *newNode(int data) { struct Node *new_node = (struct Node *) malloc(sizeof(struct Node)); new_node->data = data; new_node->next = NULL; return new_node; } /* Function to insert a node at the beginning of the Doubly Linked List */ void push(struct Node** head_ref, int new_data) { /* allocate node */ struct Node* new_node = newNode(new_data); /* link the old list off the new node */ new_node->next = (*head_ref); /* move the head to point to the new node */ (*head_ref) = new_node; } /* Adds contents of two linked lists and return the head node of resultant list */ struct Node* addTwoLists (struct Node* first, struct Node* second) { struct Node* res = NULL; // res is head node of the resultant list struct Node *temp, *prev = NULL; int carry = 0, sum; while (first != NULL || second != NULL) //while both lists exist { // Calculate value of next digit in resultant list. // The next digit is sum of following things // (i) Carry // (ii) Next digit of first list (if there is a next digit) // (ii) Next digit of second list (if there is a next digit) sum = carry + (first? first->data: 0) + (second? second->data: 0); // update carry for next calulation carry = (sum >= 10)? 1 : 0; // update sum if it is greater than 10 sum = sum % 10; // Create a new node with sum as data temp = newNode(sum); // if this is the first node then set it as head of the resultant list if(res == NULL) res = temp; else // If this is not the first node then connect it to the rest. prev->next = temp; // Set prev for next insertion prev = temp; // Move first and second pointers to next nodes if (first) first = first->next; if (second) second = second->next; } if (carry > 0) temp->next = newNode(carry); // return head of the resultant list return res; } // A utility function to print a linked list void printList(struct Node *node) { while(node != NULL) { printf("%d ", node->data); node = node->next; } printf("\n"); } /* Driver program to test above function */ int main(void) { struct Node* res = NULL; struct Node* first = NULL; struct Node* second = NULL; // create first list 7->5->9->4->6 push(&first, 6); push(&first, 4); push(&first, 9); push(&first, 5); push(&first, 7); printf("First List is "); printList(first); // create second list 8->4 push(&second, 4); push(&second, 8); printf("Second List is "); printList(second); // Add the two lists and see result res = addTwoLists(first, second); printf("Resultant list is "); printList(res); return 0; }

## Java

// Java program to delete a given node in linked list under given constraints class LinkedList { static Node head1, head2; static class Node { int data; Node next; Node(int d) { data = d; next = null; } } /* Adds contents of two linked lists and return the head node of resultant list */ Node addTwoLists(Node first, Node second) { Node res = null; // res is head node of the resultant list Node prev = null; Node temp = null; int carry = 0, sum; while (first != null || second != null) //while both lists exist { // Calculate value of next digit in resultant list. // The next digit is sum of following things // (i) Carry // (ii) Next digit of first list (if there is a next digit) // (ii) Next digit of second list (if there is a next digit) sum = carry + (first != null ? first.data : 0) + (second != null ? second.data : 0); // update carry for next calulation carry = (sum >= 10) ? 1 : 0; // update sum if it is greater than 10 sum = sum % 10; // Create a new node with sum as data temp = new Node(sum); // if this is the first node then set it as head of // the resultant list if (res == null) { res = temp; } else // If this is not the first node then connect it to the rest. { prev.next = temp; } // Set prev for next insertion prev = temp; // Move first and second pointers to next nodes if (first != null) { first = first.next; } if (second != null) { second = second.next; } } if (carry > 0) { temp.next = new Node(carry); } // return head of the resultant list return res; } /* Utility function to print a linked list */ void printList(Node head) { while (head != null) { System.out.print(head.data + " "); head = head.next; } System.out.println(""); } public static void main(String[] args) { LinkedList list = new LinkedList(); // creating first list list.head1 = new Node(7); list.head1.next = new Node(5); list.head1.next.next = new Node(9); list.head1.next.next.next = new Node(4); list.head1.next.next.next.next = new Node(6); System.out.print("First List is "); list.printList(head1); // creating seconnd list list.head2 = new Node(8); list.head2.next = new Node(4); System.out.print("Second List is "); list.printList(head2); // add the two lists and see the result Node rs = list.addTwoLists(head1, head2); System.out.print("Resultant List is "); list.printList(rs); } } // this code has been contributed by Mayank Jaiswal

## Python

# Python program to add two numbers represented by linked list # Node class class Node: # Constructor to initialize the node object def __init__(self, data): self.data = data self.next = None class LinkedList: # Function to initialize head def __init__(self): self.head = None # Function to insert a new node at the beginning def push(self, new_data): new_node = Node(new_data) new_node.next = self.head self.head = new_node # Add contents of two linked lists and return the head # node of resultant list def addTwoLists(self, first, second): prev = None temp = None carry = 0 # While both list exists while(first is not None or second is not None): # Calculate the value of next digit in # resultant list # The next digit is sum of following things # (i) Carry # (ii) Next digit of first list (if ther is a # next digit) # (iii) Next digit of second list ( if there # is a next digit) fdata = 0 if first is None else first.data sdata = 0 if second is None else second.data Sum = carry + fdata + sdata # update carry for next calculation carry = 1 if Sum >= 10 else 0 # update sum if it is greater than 10 Sum = Sum if Sum < 10 else Sum % 10 # Create a new node with sum as data temp = Node(Sum) # if this is the first node then set it as head # of resultant list if self.head is None: self.head = temp else : prev.next = temp # Set prev for next insertion prev = temp # Move first and second pointers to next nodes if first is not None: first = first.next if second is not None: second = second.next if carry > 0: temp.next = Node(carry) # Utility function to print the linked LinkedList def printList(self): temp = self.head while(temp): print temp.data, temp = temp.next # Driver program to test above function first = LinkedList() second = LinkedList() # Create first list first.push(6) first.push(4) first.push(9) first.push(5) first.push(7) print "First List is ", first.printList() # Create second list second.push(4) second.push(8) print "\nSecond List is ", second.printList() # Add the two lists and see result res = LinkedList() res.addTwoLists(first.head, second.head) print "\nResultant list is ", res.printList() # This code is contributed by Nikhil Kumar Singh(nickzuck_007)

Output:

First List is 7 5 9 4 6 Second List is 8 4 Resultant list is 5 0 0 5 6

Time Complexity: O(m + n) where m and n are number of nodes in first and second lists respectively.

Related Article : Add two numbers represented by linked lists | Set 2

Please write comments if you find the above codes/algorithms incorrect, or find other ways to solve the same problem.